How come the G15 is a 1.8-2.8 and doesn't weigh a 100lbs? I actually wouldn't mind a variable aperature zoom like the G15's. why don't they do that?

Completely different sensor. In relation to the actual question, there ARE zooms faster than f/2.8, just for other sensors. An f/2.8 lens on a medium format camera will be completely different to a 35mm camera, and different in a micro 4/3 sensor.

It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter.

Actaully, that 'rule' applies to telephoto lens designs but not all lens designs. Else, the front element of the 16-35mm f/2.8L would be only 1.25 cm in diameter.

Not to worry. The formula is pretty much correct. But there are other factors that come in to play. But as a rule of thumb it works good. It does not take in to account the convex and lens shapes required for UWA lenses for example, as pointed out. If you did make a straight 35mm by f2.8 you could theoretically get away with a 1.25cm diameter, but that's not 100% of the story. That's typical, I was just starting to get it and now you complicate things again

How come the G15 is a 1.8-2.8 and doesn't weigh a 100lbs? I actually wouldn't mind a variable aperature zoom like the G15's. why don't they do that?

The G15 has a relatively small sensor with a 4.6x crop factor, and the lens as a result has a very small imaging circle. In 35mm terms, its 6.1-30.5/1.8-2.8 lens is the equivalent of a 28-140/8.3-12.9 lens. Nothing too special. Small imaging circle lenses have been doing this for years - most camcorders with tiny 1/6" sensors have quite big zoom ranges, yet they open up to about f1.8 at the wide end.

But your question about variable aperture makes sense. These constant aperture lenses such as the 70-200's are pushing the limits at 200mm due to the entrance pupil opening up to almost the size of the front element, but down at 70 you get the impression they're held back. It probably would be possible with just some minor tweaks to make a 70-200/2.0-2.8 lens.

As around 50mm seems to be about the sweet spot for easy to make large apertures, zooms at the other end of the range such as 16-35's might be possible with f2.8 at 16mm, and getting slightly larger, again to about f2.0 at around the 35mm mark.

9.3-930mm/1.7-4.7, designed for a 2/3" sensor. That's a 3.93x crop factor to convert to 35mm terms, so its effectively a 36.5 to 3650mm lens, with an aperture varying between f6.6 and f18.4 - very impressive stuff (especially having a 100x zoom and being able to crop into an area just 1.38cm x 0.78cm with its built in 2x TC enabled), but it doesn't fit the bill as being equivalent to a FF zoom with an effective aperture bigger the f2.8.

Going bigger seems to be the only option. A lens like the Hasselblad 50-110/3.5-4.5 seems to be a good bet. In 35mm speak its f2.5 at the wide end of its focal range.

Zlatko

9.3-930mm/1.7-4.7, designed for a 2/3" sensor. That's a 3.93x crop factor to convert to 35mm terms, so its effectively a 36.5 to 3650mm lens, with an aperture varying between f6.6 and f18.4 - very impressive stuff (especially having a 100x zoom and being able to crop into an area just 1.38cm x 0.78cm with its built in 2x TC enabled), but it doesn't fit the bill as being equivalent to a FF zoom with an effective aperture bigger the f2.8.

How come the G15 is a 1.8-2.8 and doesn't weigh a 100lbs? I actually wouldn't mind a variable aperature zoom like the G15's. why don't they do that?

Most photographers have a huge misunderstanding about this issue. Simply put the bigger the sensor is the bigger the lens, and the bigger the sensor and lens, the more light you get falling onto the sensor.

That f/1.8-f/2.8 lens on the g15 equivalent both in the amount of light it gathers and apparent background blur, and in every way that matters to a full frame:

f/10-f/16 lens.

Much less impressive.

To actually be equivalent to an f/1.8 lens on full frame the G15 would have to have an f/0.3 lens, which if using the same optical design would have a 29 centimeter front filter thread (11.4 inches in diamiter). It would look something like this in dimensions:

In 35mm terms, its 6.1-30.5/1.8-2.8 lens is the equivalent of a 28-140/8.3-12.9 lens. Nothing too special.

Don't want to go off topic, but in 35mm terms it is still an f/1.8 - 2.8 lens. It might have an equivalent field of view as a 28-140mm lens. But the shutter speed is based on the f/1.8 to 2.8 aperture, which is pretty good. The equivalent background (or lack of) is what you'd expect from a 6.1 - 30.5mm lens (which is largely independent of sensor size). That is, you'd have similar background blur at 30.5mm at f/2.8 on G15 as you would with a 16-35mm zoomed to 30.5mm at f/2.8 on a 5Diii. And in dark shooting conditions, shooting macro, or when capturing action with the fastest possible shutter speed, the ability to have more in focus for a given aperture is sometimes just as important. (And besides, in post production it is easier to blur a background than add more detail in).

In a round about way, just trying to say that the aperture range of a lens should be viewed independently of sensor size. From there, most sensible photographers can use their own judgement as to whether a particular system/sensor size is suitable for their purpose. Need more background in focus, shoot m43 or the G15. Need more background blur, shoot medium format (or adjust in post production). Need it just right, shoot full frame.

Imagine this scenario - Canon releases an EF-S 400mm f/4 with image quality, build and price equivalent to the EF 400mm f/5.6. Even though some might say it is still a "f/5.6 equivalent", most people would instantly see the benefit of the faster aperture.

In 35mm terms, its 6.1-30.5/1.8-2.8 lens is the equivalent of a 28-140/8.3-12.9 lens. Nothing too special.

Don't want to go off topic, but in 35mm terms it is still an f/1.8 - 2.8 lens. It might have an equivalent field of view as a 28-140mm lens. But the shutter speed is based on the f/1.8 to 2.8 aperture, which is pretty good. The equivalent background (or lack of) is what you'd expect from a 6.1 - 30.5mm lens (which is largely independent of sensor size). That is, you'd have similar background blur at 30.5mm at f/2.8 on G15 as you would with a 16-35mm zoomed to 30.5mm at f/2.8 on a 5Diii. And in dark shooting conditions, shooting macro, or when capturing action with the fastest possible shutter speed, the ability to have more in focus for a given aperture is sometimes just as important. (And besides, in post production it is easier to blur a background than add more detail in).

In a round about way, just trying to say that the aperture range of a lens should be viewed independently of sensor size. From there, most sensible photographers can use their own judgement as to whether a particular system/sensor size is suitable for their purpose. Need more background in focus, shoot m43 or the G15. Need more background blur, shoot medium format (or adjust in post production). Need it just right, shoot full frame.

Imagine this scenario - Canon releases an EF-S 400mm f/4 with image quality, build and price equivalent to the EF 400mm f/5.6. Even though some might say it is still a "f/5.6 equivalent", most people would instantly see the benefit of the faster aperture.

People have been misled by this insane nonsense for years.

The "35mm equivalent" is what is really important and nothing else.

From a physics perspective the "35mm equivalent" is capturing identical information. What really matters is the geometry of the light hitting the sensor:

Generally the technical difficulty of achieving a particular geometry is INDEPENDENT of sensor size, meaning it's equally difficult to create a 17-55mm f/2.8 IS lens as it is to create a 24-105mm f/4.0 IS.

The front element of a 1/1.7" sensor lens that is 8-30mm f/0.3 lens would be 11.4 inches, and so would the front element of a 35mm sensor that has a 28-140mm f/1.8 lens.

Going back to the 35mm equivalent discussion, consider this:

On 7D compared the the 5D Mark III

The sensor is 1.6 x 1.6 times smaller.

35mm equivalent aperture - Multiply by (1.6 x 1.6 / 2 ) (an f stop is a base 2 log, so we divide by 2 to multiply between base 10 and base 2 if you were wondering, this just converts the number systems, nothing else)

35mm equivalent focal length - Multiply by 1.6

35mm equivalent ISO or light sensitivity - Multiply by (1.6 x 1.6) (bet you haven't heard of that, but if you do the math the 7D's sensor amplifies the signal 1.6x1.6 times more at a given ISO than the 5D3)

The point is that people are often misled by manufacturers changing the geometry of a camera system, particularly putting in small sensors and then claiming otherwise impressive performance numbers which are incredibly misleading because you are measuring them on a different scale.

It's like saying:

I have a million dollars, and then failing to mention these are Zimbabwe dollars worth $20 not, American dollars.

Yes aperture ISO and focal length are fixed numbers, but so are monetary figures, and the most important thing even the most basic dealing of currency has is WHAT currency you're dealing with, and 99% of people require an "equivalent" frame of refference to understand foreign currency or need to do a conversion. Likewise with cameras, geometry (type of currency) is the most important thing when dealing with the performance of a camera system, and the first thing anyone needs to do is bring up a conversion to the local frame of reference, APS-C 35mm, whatever.

To respond to your post though, there is NO benefit to a 1 stop faster aperture on APS-C sensor vs full frame because they (more than) cancel each other out. You don't stop action any quicker at all, whatsoever, because remember the ISO is skewed too, so all you're doing is just turning up the ISO sensitivity in a roundabout way. You have been misled into thinking there is a benefit.

Is it a business decision (ie dont want to canabalize the prime market) or a technical limitation (physically can't do it with current tech) or cost issue (technical difficulties would require that the lens be priced too high for target market)?

There are hard physics facts which limit the chance to use well known designs to built such a lens.

Canon and all the other lens producers use exotic materials like high refractive index glass or lenses with special dispersion properties in their premium lenses. There is no headroom to change glass types. Let's dream about a glass type which has a refractive index of 2.5 or 3 without any dispersion and a super coating which increases the transmission to 99.9 %!

Than you have to recalculate the whole design including the 4, 5 or 6 lens groups which are positioned during zooming! This might introduce another 5 or 10 lenses to achieve good IQ and than you will loose more light due to reflections on the lenses ...

There are no physics killer facts to built a e.g. f/2.0 70-200 but physics will reduce IQ and/or increase massively the size (not only front element diameter but also the length). If you demand the IQ of the 2.8 70-200 II you will pay sth. between 50000 and 200000 $/€ for that lens - if it is possible to get such a high IQ. And than your third point factors in ...

In 35mm terms, its 6.1-30.5/1.8-2.8 lens is the equivalent of a 28-140/8.3-12.9 lens. Nothing too special.

Don't want to go off topic, but in 35mm terms it is still an f/1.8 - 2.8 lens. It might have an equivalent field of view as a 28-140mm lens. But the shutter speed is based on the f/1.8 to 2.8 aperture, which is pretty good. The equivalent background (or lack of) is what you'd expect from a 6.1 - 30.5mm lens (which is largely independent of sensor size). That is, you'd have similar background blur at 30.5mm at f/2.8 on G15 as you would with a 16-35mm zoomed to 30.5mm at f/2.8 on a 5Diii. And in dark shooting conditions, shooting macro, or when capturing action with the fastest possible shutter speed, the ability to have more in focus for a given aperture is sometimes just as important. (And besides, in post production it is easier to blur a background than add more detail in).

In a round about way, just trying to say that the aperture range of a lens should be viewed independently of sensor size. From there, most sensible photographers can use their own judgement as to whether a particular system/sensor size is suitable for their purpose. Need more background in focus, shoot m43 or the G15. Need more background blur, shoot medium format (or adjust in post production). Need it just right, shoot full frame.

Imagine this scenario - Canon releases an EF-S 400mm f/4 with image quality, build and price equivalent to the EF 400mm f/5.6. Even though some might say it is still a "f/5.6 equivalent", most people would instantly see the benefit of the faster aperture.

As Radiating pointed out, these smaller sensors have their amplification turned up to make their ISO ratings match the apertures. ISO 1600 on a G15 at f2.8 and 1/100th of a sec will match ISO 1600 on a 5D3 at f2.8 and 1/100th of a sec for exposure. But I'm sure no-one here disputes the fact that the smaller G15 sensor will produce much more noise at the same ISO's. Its simply because the amplification is turned up much higher to compensate for the much smaller imaging circle projecting much less light onto the sensor.

If the imaging circle is a fraction of the size, only a fraction of the light is projected onto the sensor.

While it is a 6.1-30.5/1.8-2.8 lens, it only covers a small imaging circle. To put it into context of a FF setup, it is factually incorrect to only scale up the focal length. The whole system has to get scaled up - so it becomes equivalent of a 28-140/8.3-12.9 lens in terms of focal length, DoF and quantity of light. And to complete that, you should also factor in what the sensor does - its ISO 80 to 12,800 sensor is the equivalent of a FF sensor which operates between ISO 1600 and 270,000.

I wouldn't get involved with blurring backgrounds in post, it will never look right. Instagram will never replace a good lens on a DSLR - just take the photo with the right camera and the right aperture. If you need more DoF with a FF setup, stop the lens down. Shooting at f32 will cause all sorts of diffraction issues, but so does f6.9 on a compact with an equal number of MP. You can always crank up the ISO and NR, and then set the sharpness to 10 in true compact style to ensure no fine detail remains.

Canon already have a 400/4 lens out there - its a FF DO lens, and costs five times as much as the f5.6 version.

As Radiating pointed out, these smaller sensors have their amplification turned up to make their ISO ratings match the apertures. ISO 1600 on a G15 at f2.8 and 1/100th of a sec will match ISO 1600 on a 5D3 at f2.8 and 1/100th of a sec for exposure. But I'm sure no-one here disputes the fact that the smaller G15 sensor will produce much more noise at the same ISO's. Its simply because the amplification is turned up much higher to compensate for the much smaller imaging circle projecting much less light onto the sensor.

The "amplification" is not "turned up" for the smaller sensor. A 6x4mm sensor is no more noisy (or "amplified") than a 6x4mm region of a 35mm sensor. Images from the smaller sensor are (theoretically) about as noisy as a crop from a larger sensor.

The reduction in noise has to do with sampling density and averaging, not amplification.